After Nazi brainwashing they see people as crud and kill humans like scuzzy animals.

It was beaten into their emty heads that people who antagonize against Americans must be crud, dirty shit, rubbish, just for the birds. The outcome of this is the bestial perception of a Nazi slaughterer and it had become easy and funny to slaughter humans such as 'jewish filthy pigs' in former times

Same methods - Same reason - The Pest has a name